From the past, Silence in a motor is considered. There exist many techniques which intend to achieve silence by paying attention to the variation in attraction force in rotational direction due to rotational position such as cogging torque, mainly.
For example, Japanese Patent Publication Nos. 10-201147, 11-98731, and 2000-69695 each intends to reduce cogging torque.
Japanese Patent Publication No. 10-201147 realizes a motor with low vibration and low noise by reducing cogging torque by determining circumferential direction target position of permanent magnets in two layers to be a specific relationship which suits to a specific formula, the permanent magnets being embedded in the interior of the rotor core, as illustrated in FIG. 1.
Japanese Patent Publication No. 10-201147 pays attention to cogging torque, so that formula differs depending upon the cycle of cogging torque. Especially, the value of θ in FIG. 1 becomes greater when concentrated winding is employed, so that disadvantage arises in that permanent magnets disposed in outer peripheral side of the rotor cannot be made greater.
Further, an angle of aperture of a magnetic pole differs for every pole, so that unbalance is generated in rotational force. And, an induction voltage waveform becomes asymmetrical shape. Therefore, various abuse are expected such that increase in noise, affection in position detection accuracy and the like due to other cause.
Japanese Patent Publication No. 11-98731 intends to reduce cogging torque by varying the angle of aperture of the magnetic pole for every pole, as illustrated in FIG. 2.
But, an angle of aperture of a magnetic pole differs for every pole, so that unbalance is generated in rotational force. And, an induction voltage waveform becomes asymmetrical shape. Therefore, various abuse are expected such that increase in noise, affection in position detection accuracy and the like due to other cause.
Japanese Patent Publication No. 2000-69695 determines the punching holes for embedding permanent magnet which are equal to one another, and varies the shape of elongate hole section each provided to contact to the edge face of the permanent magnet. This corresponds to application of pseudo skew, and intends to reduction effect in cogging torque.
Piling different rotor cores in axial direction causes complicated arrangement of die. Force is generated in axial direction so that thrust vibration is possibly induced. Abuse cannot be ignored due to existence of pole which has extremely narrow angle of aperture in magnetic pole. For example, problems are exemplified such that increase in iron losses due to excessive concentration of magnetic flux, sufficient magnetic flux cannot reach edge section of permanent magnet during magnetization.
All of those techniques pay attention to cogging torque. When a motor is driven, more particularly when load is great, affection of cogging torque is small, and rather, affection of higher harmonics of induction voltage becomes greater if cogging torque is suppressed below a certain value. More particularly, when permanent magnet having stronger magnetic force is employed such as rare earth magnet, it is understood that the magnet is greatly affected by higher harmonics of induction voltage.
Japanese Patent Publication No. 2002-44888 can resolve the problem of the existence of extremely narrow angle of aperture in magnetic pole among the above problems. Specifically, an elongate hole section provided for contacting the edge face of the permanent magnet is divided in peripheral direction, as illustrated in FIG. 4, so that “auxiliary reinforcement rib” is provided. Japanese Patent Publication No. 2002-44888 intends to prevent deformation in the rotor.
The magnetic flux also flows through the “auxiliary reinforcement rib”. Therefore, sound may be increased depending upon the position of the “auxiliary reinforcement rib”.
As is described in the above, Japanese Patent Publication Nos. 10-201147, 11-98731, 2000-69695, and 2002-44888 do not successfully reduce cogging torque while sufficient permanent magnet magnetic flux is maintained and abuse in sound due to unbalance vibration and in sound due to thrust force is not generated. Also, they are not the measure which pays attention to specific harmonics of the induction voltage, for example fifth order harmonics and seventh order harmonics. Especially, they are not the arrangement for sufficiently reduce sound when the motor has a great load.
The present invention was made in view of the above problems.
It is an object of the present invention to provide a motor for reducing specific orders of magnetic flux distribution waveform, for example fifth order and seventh order, and for prevent generation of unnecessary radial force and thrust force while sufficient magnetic flux is maintained.